As microfabrication of semiconductor devices has advanced, pattern sizes have decreased less than one-several tens of a wavelength of light, thus revealing a limit of resolution in inspection using an optical microscope. A scanning electron microscope (SEM) can observe patterns with a high resolution because the wavelength of electrons is short as compared with the pattern sizes. Therefore, the scanning electron microscope has been widely used for semiconductor inspection. In recent years, however, patterns have become smaller and finer, and accordingly, a demand for the resolution of the scanning electron microscope has been increasing year by year. Furthermore, there is a demand for high-speed scanning over a large area in order to meet a requirement of a high throughput. Therefore beam current of 1 nA or more is becoming indispensable for obtaining a sufficient SN ratio in a short time.
Generally, the higher an acceleration voltage for a primary beam increases, the higher a brightness of a light source becomes, thus achieving a higher resolution of the scanning electron microscope. However, the high acceleration voltage results in heavier damage to devices, besides a problem of shrinkage when observing resist patterns, it has been required to observe semiconductor devices with a landing energy of 5 keV or lower. In the scanning electron microscope, if the acceleration voltage is low, a coulomb interaction occurs noticeably depending on the increase of the beam current. As a result, the coulomb interaction in a beam path causes the primary beam to spread more greatly than an optical aberration, thus lowering the resolution.
There are strong needs, as technical tasks, for a future pattern observation to overcome the above-discussed drawbacks and to achieve the following five objects simultaneously, i.e., 1) a low landing energy of at most 5 keV, 2) a high resolution, 3) a high beam current, 4) high-speed scanning, and 5) a large field of view (FOV) of 100 μm.
Multilayered patterns have also been advancing, as well as the microfabricated patterns. With this trend, there are also strong demands of observing minute positional misalignment of patterns between upper and lower layers and observing a bottom of a contact hole having a high aspect ratio. In order to accommodate these needs, it is necessary to use an acceleration voltage enabling a high penetrating power and to observe back-scattered electrons having appropriate energies determined depending on a depth from a surface of an observation target. For realizing this, it is effective to observe not only secondary electrons, but also back-scattered electrons whose amount of energy loss has been specified.
The present invention has been made to provide a scanning electron microscope capable of achieving the following seven objects, which are:
(1) to use a low landing energy of not more than 5 keV which can avoid damage to semiconductor devices;
(2) to achieve a large FOV of 100 μm;
(3) to achieve a high-speed scanning of 100 MHz or higher;
(4) to use a large beam current of at least 1 nA capable of achieving a sufficient SN ratio (signal-to-noise ratio) even at a pixel rate of 100 MHz;
(5) to observe a target object with a probe size of not more than 2 nm which can detect a defect having a size of several nanometers;
(6) to use a high acceleration voltage of 50 kV to increase a depth by which a primary beam can penetrate into a specimen for enabling acquisition of information carried by back-scattered electrons; and
(7) to specify a depth at which the back-scattered electrons (BSE) are generated by selecting a detection energy for the back-scattered electrons (BSE) in order to enable selection of the specified depth information.
The prior art related to a method of detecting the secondary particles includes laid-open patent publications U.S. Pat. Nos. 7,462,828, 8,153,969. According to these documents, as shown in FIG. 17, an electron beam 200 is illuminated to a specimen 203 transmitting through a condenser lens CL1, an aperture 201, a condenser lens CL2, and an objective lens 202. A target plate 205, which is grounded, is disposed on a beam axis. Secondary particles collide with the target plate 205, so that secondary electrons (which may be referred to as tertiary electrons) are emitted from the target plate 205. The secondary electrons are attracted by an attraction force of an electrostatic field created by applying a positive voltage of about 10 KV to a scintillator 206 which is arranged far away from the beam axis, and are detected by the scintillator 206.
The detection method using the target plate 205 generally has three disadvantages.
The first disadvantage is that when the energy of the secondary particles (i.e., the secondary electrons and the back-scattered electrons), which enter the target plate 205, exceeds about 2 KeV, a secondary electron yield (which may to be referred to as tertiary electrons) of the target plate 205 becomes smaller than 1. For example, when the energy of the secondary particles is 10 keV, the secondary (tertiary) electron yield decreases to 0.5 or less. The tertiary electrons are directed to the scintillator 206, causing the scintillator 206 to emit light. The light is reduced while traveling through a light guide 207. Further, the light is transmitted to a photo-multiplier tube (PMT) 208, where the light is converted into electrons and multiplied. Through these processes, the SN ratio considerably deteriorates, as discussed below.
Target-type signal detection processes include conversion processes in several stages. First, the secondary electrons, which are signal sources, collide with the target plate 205, and tertiary electrons are emitted. The tertiary electrons are accelerated and enter the scintillator 206, where the tertiary electrons are converted into light. The light penetrates through the light guide 207 to reach the PMT 208, where the light is converted into electrons again. These electrons are cascade-multiplied. FIG. 18 is a graph showing SN-ratio deterioration factor in these signal detection processes. A symbol F1 represents a noise factor in a process in which the tertiary electrons, which have been emitted from the target plate 205 by the incident secondary electrons, are converted into the light, and a symbol F2 represents a noise factor in a process in which the light, generated in the scintillator 206, is converted into the electrons by the PMT 208. The SN ratio of an output is expressed by(S/N)out=(S/N)in×(1/F1)×(1/F2)
where δ2 represents a tertiary electron yield of the target plate 205, σ represents probability that the tertiary electrons reach the scintillator 206, Nq represents the number of photons emitted from the scintillator 206 by the secondary or tertiary electrons, and Pc represents probability that the photons reach the PMT 208 to cause the PMT 208 to emit photoelectrons and to successfully accomplish cascade amplification at a subsequent stage. In a case where the secondary electrons, which are signal sources, are detected directly by the scintillator, only F2 is taken into account.
FIG. 19 is a graph showing the tertiary electron yield δ2 in the target-plate detection method. As shown in FIG. 19, when the energy of the secondary particles is as high as 10 KeV, the tertiary electron yield δ2 is reduced to about 0.5. As a result of the fact that δ2 is reduced to about 0.5 even if σ has a good value of near 1, the SN ratio is deteriorated in the conversion process because the SN ratio is multiplied by a 1/F1 factor which is about 0.5, as shown in a left graph of FIG. 18.
A right graph of FIG. 18 shows a noise factor in the process in which the light is converted into electrons. Since PcNq can have a value in a range of about 3 to 6, a value of 1/F2 is about 0.9, and as such, the deterioration of the SN ratio in this process is not so bad. In this manner, in the target method, the process of the tertiary electron emission from the target plate 205 is a bottleneck, which hinders the method from obtaining a good SN ratio.
The second disadvantage is that, since an initial energy of the tertiary electrons generated from the target plate 205 is as low as several eV, the tertiary electrons trace variously curved trajectories depending on differences in their generation point and their take-off angle besides azimuthal angle until they reach the scintillator 206, as shown in FIG. 17. According to simulation results, there is a time difference of about 20 ns at most in the transit time, as a result of a difference in length of the trajectories and the low speed of the tertiary electrons. When high-speed scanning is performed at 100 MHz, this width of the transit time is several times as long as a primary-beam dwelling time of 10 ns per pixel, thus causing an image blur in the scanning direction. For this reason, the target method is not suitable for the high-speed scanning.
The third disadvantage is that a high voltage of about 10 KV is applied to the scintillator 206 of this target type in order to draw or attract the low-speed tertiary electrons. The application of the high voltage generates an electric field, which is likely to cause a static beam deflection of the primary beam and it tends to fluctuate.
In order to solve the above-described problems, a scintillator arranged on the beam axis may be used, instead of using the above-described target plate 205 arranged on the beam axis. Direct detection by the scintillator is free from the above-described bottleneck and provides a detection system which is easy to attain a good SN ratio. The laid-open patent publication U.S. Pat. No. 8,895,935 is available as a prior patent document about an on-axis scintillator. The scintillator disclosed in this document is a detection system with a single-stage configuration. In order to enable the scintillator to be disposed on the beam axis, a metal tube for the passage of a primary beam is provided. However, no measure is taken for secondary particles escaping upstream through the metal tube. This document discloses another example wherein the secondary particles are deflected by an EXB (E cross B) so that the secondary particles are detected by a scintillator which is disposed away from the beam axis. In this case, it is necessary for the EXB to have a wide opening in order to deflect and detect all of the secondary particles which are spreading widely over a large area. This configuration entails a high voltage application and a large magnetomotive force. Namely, it is necessary to strongly excite the EXB in order to deflect a wide flux of all of the secondary particles out of the axis. As a result, the EXB has a strong influence on the primary beam accordingly. For these reasons, it is extremely difficult to capture all of the secondary particles with the single-stage scintillator in an SEM aimed at a large FOV.
Thus, there is proposed a detection system having scintillators of two-stage configuration. The two-stage scintillators are configured to detect widely-spreading secondary particles with an initial-stage on-axis scintillator, to deflect secondary particles that have passed through a central hole of the initial-stage on-axis scintillator with EXB (E cross B), and to detect the deflected secondary particles with a subsequent-stage scintillator. However, as the EXB affects the primary beam in a manner to increase a beam diameter, a certain measure for avoiding such an effect is indispensable for realizing a high-resolution SEM.
The above-described laid-open patent publications U.S. Pat. Nos. 7,462,828 and 8,153,969 disclose a measure for avoiding the adverse effect that the EXB has on the primary beam. According to these publications, two sets of EXBs are disposed on the axis, and are excited in mutually opposite polarities so as to cancel out energy dispersions and aberrations which are generated in these sets of EXBs. However, this arrangement is not practical because of disadvantageous conditions imposed this system, i.e., (a) two sets of EXBs are necessary, resulting in high costs, (b) due to limitations of their mutual axial precisions, it is difficult to conduct adjustment such that the primary beam travels straight, (c) the total length of the system becomes long because of the increased length of the optical path of the primary beam, and (d) the incident primary beam has to be parallel to the two sets of EXBs in order for them to cancel out the aberrations completely.
Another measure for avoiding the drawback is disclosed in the laid-open patent publication U.S. Pat. No. 6,455,848, which describes a method of reducing aberrations by focusing the primary beam to form a crossover in the center of EXB. However, this publication discloses nothing about a divergence angle of the crossover. Under a certain beam current condition, there is an optimum value for the divergence angle that minimizes the beam diameter on a specimen surface, and the optimum value depends on the beam current. Consequently, simply forming the crossover at the center of EXB is not sufficient for achieving an optimum condition for various beam current values, therefore the crossover has to be fixed in position and the divergence angle thereof also has to be controlled at an optimum value in accordance with the current value.